Variable optical retarders are used to manipulate polarization and phase properties of optical beams. Liquid crystal materials are frequently used for this purpose due to large electro-optical coefficients of liquid crystal fluids. In a liquid crystal variable optical retarder, a voltage is applied to a thin layer of a liquid crystal fluid comprised of oriented liquid crystal molecules. The molecules align relative to the electric field due to induced electrical dipole interaction with an electric field of the applied voltage, changing effective refractive index of the liquid crystal layer and thus changing a delay or phase of a polarized light beam propagating through the layer. When the light beam propagates through a two-dimensional array of such liquid crystal variable optical retarders, the spatial polarization or phase distribution of the light beam changes in accordance with distribution of individual voltages applied to individual retarders of the array.
Although liquid crystal arrays have been originally developed primarily for information displays, they have been finding a steadily increasing use in optical networking equipment, such as dynamic gain equalizers for equalizing spectral gain of optical amplifiers, wavelength blockers for selective blocking wavelength channels and, more recently, in wavelength selective optical switches (WSS). WSS operate to independently switch individual wavelength channels between different fibers of fiberoptic communications networks.
Frisken in U.S. Pat. No. 7,092,599 discloses a wavelength selective switch having a liquid crystal array as a switching element. The liquid crystal array is driven by AC voltages of different phases and frequencies, for example, 1 kHz, 2 kHz, 4 kHz, and 8 kHz, applied directly to different row and column electrodes of the liquid crystal (LC) array. One drawback of directly driven liquid crystal arrays is a reduced number of optical retardation levels (called “grayscale levels” in information display industry), and a relatively slow response of the LC fluid. The slow response of the LC fluid is required to avoid time domain modulation, or flicker, due to the multi-frequency AC modulation used to generate the grayscale levels.
Active matrix liquid crystal arrays allow for faster operation, with more optical retardation levels attainable. In an analog active matrix liquid crystal array, a dedicated electrical switch or gate element is connected to, and disposed next to, each optical retarder element of the array. The gate element can be opened by applying an external gate voltage to a gate bus electrode, which allows the liquid crystal retarder to store an electric charge when a corresponding signal voltage is simultaneously applied to a signal bus electrode crossing the gate electrode at the gate element's location. The stored electrical charge generates a constant voltage across the retarder element, defining its optical retardation value until next data writing sequence.
Among different active matrix liquid crystal array implementations, reflective liquid crystal arrays disposed on a silicon substrate (“Liquid Crystal on Silicon” or LCoS) are of a particular interest. The advantage of LCoS arrays is that the gate elements and/or other driver circuitry can be conveniently disposed on the silicon substrate behind the liquid crystal layer, resulting in a large fill factor of the LCoS arrays, of about 90%. This makes LCoS arrays promising switching elements for WSS applications.
Frisken et al. in U.S. Pat. No. 7,457,547 disclose a LCoS-based WSS device. Referring to FIG. 1, a LCoS-based WSS 10 includes an input port 12, wavelength dispersing an collimating optics shown as a dashed rectangle 14, a LCoS array 16, and a plurality of output ports 18. The LCoS array 16 includes a silicon substrate 20 having thereon some driving circuitry, not shown, pixel electrodes 22, a liquid crystal layer 24, and a Indium Tin Oxide (ITO) transparent backplane common electrode 26. In operation, the LCoS array 16 is driven by applying analog voltages to the individual pixel electrodes 22, to create a saw tooth optical retardation profile 28, which acts as a reflective phase diffraction grating, in which the periodicity of the grating determines the steering angle, and the height h of the profile the amount of power that is coupled into the first diffraction order. The saw tooth optical retardation profile 28, defining a corresponding linear optical retardation profile 29, has a property of steering a reflected optical beam 32 to one of the output ports 18, depending on the periodicity and a slope α of the saw tooth profile 28. The LCoS array 16 is driven to vary the periodicity and/or the slope α of the saw tooth optical retardation profile 28, which causes the reflected optical beam 32 to steer in space and to couple into a desired one of the output ports 18. Detrimentally, when the saw tooth profile 28 ceases to be linear due to local variations of optical retardation, aging, or temperature change, a time domain modulation (TDM) of the reflected wavelength channel optical beam 32 can occur upon coupling of the reflected optical beam 32 into the output port 18. This happens because a non-linear saw tooth profile causes an extra optical loss and, at a higher optical loss, TDM sensitivity typically increases.
Liquid crystal arrays can also be operated by applying a binary level voltage of a varying duty cycle to the liquid crystal layer. The modulation period of the binary level voltage is typically selected to be smaller than a response time of the liquid crystal layer, which then tends to integrate the applied voltage, reacting to a net voltage proportional to the duty cycle. This driving method of liquid crystal arrays is commonly referred to as “digital driving”. The digital driving, when implemented in LCoS arrays, has advantages of simplified driver circuitry, improved switching speed, and ability to control larger number of optical retarders, or pixels, in comparison with other types of liquid crystal arrays.
The above advantages of digitally driven LCoS arrays can make them highly desirable for WSS applications. However, the above mentioned TDM problem gets even worse in a digitally driven LCoS-WSS than in the analog-driven WSS device 10 described above. In a digitally driven LCoS-WSS, a driving frame rate component of TDM can be quite strong, which, while tolerable in some information display applications, can be highly detrimental in WSS applications requiring stable, controllable, and time-invariant optical throughput. Increasing the response time of the liquid crystal layer 24 can help one to alleviate the problem, but slower LC fluid increases the switching time of the WSS beyond acceptable limits, negating one of the key advantages of the digital driving.